Characteristics of laser light
The wavelengths of light emitted by the two main types of lasers are: 632.8 nm (red) for the HeNe laser and 514.5 nm (green) for the Argon laser. The power output of a standard HeNe laser ranges from 0.5 to 50 ml (the power supply is 10,000 times larger). Compared to conventional light sources, such as high pressure xenon or mercury lamps, the light output is surprisingly low, in fact, due to the collimation of the beam, even the smallest 0.5 mW laser has a brightness which is several orders of magnitude higher than that of conventional sources. A HeNe laser beam is visible and can be easily identified at a distance, even in daylight. Argon ion lasers have a light output in the order of W (and power supply in the order of kW) and require a circulation of cooling air or water.
The light from a conventional source cannot really be focused: a lens creates an image of the source in its focus according to the laws of geometrical optics and hence the power density of the image will be limited (Figure 4.5). A laser beam consists of parallel rays that are focused by a lens in a very small spot with high brightness: the relationship between the light power density in the spot and in the laser beam increases with the square of the inverse ratio of diameters; in a lens with a small focal length, this increase in light density can be of several orders of magnitude.
Focusing of an incoherent light source and of a laser beam